1. Technical Field
This invention relates to a circuit for driving a solenoid which can shorten a process of induced electromotive force generated in a solenoid coil and shorten a fall time of an inflow current to the solenoid coil when the solenoid coil, as part of a motor, an electromagnetic pump, or other device is driven by supplying DC pulses.
2. Description of the Related Art
A fundamental circuit for a DC pulse driver that drives a motor, an electromagnetic pump and so on by using electromagnetic power generated by supplying electricity to the solenoid coil is shown in FIG. 4. This circuit is such that a solenoid coil P of an electromagnetic pump is connected to a collector side of a transistor Tr1 for switching. A diode D1 for refluxing is connected in parallel to the solenoid coil P and in a reverse direction, so that the transistor Tr1 turns on. Consequently, an electric current Ip flows into the solenoid coil P when an input voltage is supplied to a base of the transistor Tr1, as seen in FIG. 5 (a). At the same time, a collector voltage Vc of the transistor Tr1 is decreased from Vcc to about zero, the current Ip flowing into the solenoid coil P is increased transiently as seen in FIG. 5 (c), and electromagnetic energy is stored in the solenoid coil P by the current Ip. When the input voltage to the base of the transistor Tr1 becomes zero, according to a self-induced electromotive force (c.f. with the equations below):
xe2x80x83e=L(xcex94Ip/xcex94t) e=L(dIp/dt)
by the electromagnetic power that makes the current flow in a direction such that change of magnetic flux is prevented, so that the electric potential of Vc is increased. Namely, the result is that a large reverse voltage is generated across the solenoid coil P. The large voltage generated across the solenoid coil P is eliminated by electric current flowing into the diode D1 that is connected in parallel to the solenoid coil P.
Accordingly, as shown by FIG. 5 (c), in the prior circuit, when the transistor Tr1 turns on, the current flowing into the solenoid coil P rises and increases according to a specific time constant. Furthermore, when the transistor Tr1 turns off, the current flowing into the solenoid coil P with its inductance becomes zero. Consequently, the magnetic flux of the solenoid coil P is decreased, so that the induced electromotive force is generated while a current Id flows into the diode D1 for refluxing. The current Id becomes zero after a specific time tf according to a relatively long time constant because of a decrease in the amount of the induced electromotive force.
A fall time of the current Ip flowing into the solenoid coil P is most rapid when nothing is connected to the ends of the solenoid coil P. However, when nothing is connected to the ends of the solenoid coil P, a voltage from several times to several tens of times of a rated voltage of a transistor (FET""s, JFET""s, MOSFET""s, etc.) used as a switching element is generated. Since such voltages can destroy the switching element, it is necessary to connect the diode D1 for refluxing as shown in FIG. 4 to protect the switching element.
However, in such a method, when a driving frequency of an input voltage increases or an off-time in a supplying pulse is reduced, the time it takes the current Ip to go to zero, tf, becomes short. This shortening of the time tf results in an undesirable condition since the input voltage rises and the current starts to increase before the current completely reaches zero, thereby decreasing an effective amplitude of the pulse as seen FIG. 5 (c). As a result, in the case of the electromagnetic pump, a plunger starts the next movement before the plunger completely returns to its initial position, so that an effective stroke of the plunger is decreased (decreasing an effective amplitude). Therefore, one method to alleviate this problem is to connect a resistor in series with the diode D1 for refluxing. Alternatively, a Zener diode (not shown in the attached figures) could be used to decrease the fall time of the current. But, in this case, heat generated by the resistor or the Zener diode in the circuit becomes a problematic.
Accordingly, as shown in FIG. 6, it is proposed that a capacitor C1 should be connected in series to an anode of the diode D1 for refluxing along with a discharge circuit for the capacitor C1 that includes a series connected diode D2 and transistor Tr2 connected in parallel to a series connected refluxing diode D1 and capacitor C1. However, in this circuit, it is desirable that an electrical charge stored in the capacitor should be discharged rapidly. Therefore, it becomes necessary to construct a more complicated circuit in consideration of a discharge timing by the transistor Tr2 along with other associated effects.
Accordingly, the present invention is to provide a circuit for driving a solenoid which can decrease a negative voltage generated across a solenoid coil and can shorten a fall time of a current flowing into the solenoid coil with a simple construction.
The present invention is a circuit for driving a solenoid coil comprising a switching element connected in series to a solenoid coil and turned on or off by an input voltage with a specific pulse width to intermittently generate magnetism in the solenoid coil. A series circuit is connected in parallel to the solenoid coil. The series circuit includes a diode and a capacitor connected in series. The diode has a cathode connected to a power source line and the capacitor is connected in series to an anode of the diode. A light-emitting diode is connected in parallel to the capacitor.
Therefore, when a condition of the switching element changes from on to off, a large negative voltage is generated across the solenoid coil by an induced electromotive force generated on the solenoid coil. But the diode is arranged in a forward direction with respect to the negative voltage, so that electric current flows into the capacitor and the light-emitting diode and the capacitor is charged. When a potential difference across the solenoid coil becomes zero, an electrical charge in the capacitor flows into the light-emitting diode until a potential difference across the capacitor becomes zero, so that the negative voltage across the solenoid coil can be quickly removed. Furthermore, because the current is refluxed rapidly, the fall time of the current flowing into solenoid coil is less compared with existing systems. Furthermore, by using the light-emitting diode in the circuit, the current flowing into the light-emitting diode is changed not to the heat but rather to the light. The resulting effect is to prevent the generation of heat in both the circuit and in the solenoid coil itself.
Moreover, a plurality of light-emitting diodes may be connected in parallel to the light-emitting diode. Therefore, inexpensive light-emitting diodes with low currents ratings (i.e. the amount of current allowed to flow from an anode to a cathode thereof) can be used advantageously to reduce cost and improve performance.
Furthermore, a protective diode may be reverse connected (according to polarity) and in parallel to each light-emitting diode. Therefore, when a voltage over a specific value is applied to the light-emitting diode, a current flows into the diode for protection to protect the light-emitting diode. As a result, an inexpensive light-emitting diode with a low current rating can be used for this purpose.
Moreover, it is desirable that a resistor with a specific value is connected in series to the capacitor and another resistor with a specific value is connected in series to the light-emitting diode. Therefore, it is possible to manipulate a charging time and a discharging time of the capacitor, and a voltage applied to each diode used for refluxing or a voltage applied to each light-emitting diode.
Furthermore, it is desirable that the light-emitting diode can be operated in any portion of an electromagnetic spectrum (e.g. visible, infrared, etc.).